Page 30

Note: Care must be taken so that macro expansion for legitimate
E-Mail does not exceed the 63-character limit on DNS labels. The
localpart of E-Mail addresses, in particular, can have more than 63
characters between dots.

Note: Domains should avoid using the "s", "l", "o", or "h" macros in
conjunction with any mechanism directive. Although these macros are
powerful and allow per-user records to be published, they severely
limit the ability of implementations to cache results of check_host()
and they reduce the effectiveness of DNS caches.

Implementations should be aware that if no directive processed during
the evaluation of check_host() contains an "s", "l", "o", or "h"
macro, then the results of the evaluation can be cached on the basis
of <domain> and <ip> alone for as long as the shortest Time To Live
(TTL) of all the DNS records involved.